High-pressure discharge lamps having a ceramic discharge vessel contain fillings which, besides a noble gas, such as, for example, argon or Xe gas, also comprise metal halide salt mixtures such as NaCe, NaTl, NaSc, and NaTlDy halide, for example, iodide or combinations of these salts. These metal halide salt mixtures are applied to obtain, inter alia, a high lamp efficacy, a specific color temperature and a specific value of the general color rendering index Ra.
High-pressure discharge lamps of this type generally have a discharge vessel which encloses a discharge space comprising the filling of the metal halide salt mixtures. The discharge space further comprises electrodes between which a discharge is maintained. Typically, the electrodes are connected to lead-through conductors, also referred to as feed-through conductors, which pierce the discharge vessel. To connect the lead-through conductors to the discharge vessel and seal it, a glass material, also known as frit, is generally used. However, due to the relatively low melting temperature of the frit and the relatively high temperatures at the discharge space of the discharge vessel when the high-pressure discharge lamp is in operation, the discharge vessel comprises extended plugs in which the frit seals the electrode lead-through conductors to the discharge vessel.
An alternative embodiment of the high-pressure discharge lamp is known from PCT patent application WO 2005/124823. The known high-pressure discharge lamp has a discharge vessel comprising a first and a second closing construction at respective sides of the discharge vessel. The closing constructions are connected to the discharge vessel and comprise a respective first and second current feed-through, at least the second of which comprises a tube having a sintered bond to the extended ceramic plug forming the second closing construction. The tube, which consists of a metal chosen from molybdenum, rhenium, tungsten, iridium, their alloys, and optionally also comprises vanadium and/or titanium, encloses a current-supply conductor while maintaining a capillary space. The tube and the current-supply conductor are welded together at an external end of the extended ceramic plug, which weld constitutes a hermetic seal of the capillary space. The known high-pressure discharge lamp has the drawback of a rather complex closing construction and a relatively short lifetime.
A further known lamp construction is described in EP1580797. This lamp has a lead-through construction of at least one ball-shaped piece made of metal chosen from the platinum group and being sealed to a ceramic plug by means of a solder.
This known construction has a number of drawbacks. During the sealing process, the solder tends to run down outside the sealing area and over the electrode itself. The solder mass thus present inside the discharge space enclosed by the discharge vessel contaminates the filling of the discharge space, which adversely affects the light properties of the lamp and thus has a detrimental effect on its lifetime.
Furthermore, the ball shape is disadvantageous because it presents problems when the volume bounded by the ceramic plug and the lead-through element is completely filled. This is all the more true when the lead-through element is composed of a row of two or more ball-shaped pieces.
Moreover, it is a drawback that there is no suitable solder that can form a strong bond with the ceramic plug and the metal of the lead-through element and also withstands the lamp operating conditions for a lamp life span of more than 1000 hours.